"Those who approach the sacrament of Penance obtain pardon from God's mercy for the offense committed against Him and are, at the same time, reconciled with the Church which they have wounded by their sins and which, by charity, by example, and by prayer labors for their conversion." (CCC, 1422)
The Act of ContritionAfter this, the priest will ask the penitent to make a good Act of Contrition.
The following is one example of such a prayer:O my God,I am heartily sorry for having offended you,and I detest all my sins,because of your just punishment,but most of all because they offend you, my God,who are all good and deserving of all my love.I firmly resolve, with the help of your grace,to sin no more,and to avoid the near occasion of sin.Amen.